1. Field of the Invention
This invention relates to thermal cracking of hydrocarbon. More specifically, the invention relates to a process and system for quenching cracked effluent with hydrocarbon feed and cracking the hydrocarbon quench material.
2. Description of the Prior Art
At present several processes exist for thermally cracking hydrocarbons to produce olefins. In one process fired heaters are used to provide the requisite heat for reaction. The reactant flows through a plurality of coils within the fired heater, the coils being arranged in a manner that maximizes the rate of heat transfer to the hydrocarbon flowing therethrough. An example of a conventional process is shown by U.S. Pat. No. 3,487,121 (Hallee). In another process heated solids are mixed with the hydrocarbon feed in a reactor. Regardless of the process used, cracked effluent must be rapidly quenched to remove excess heat thereby terminating the cracking reactions. Both direct and indirect quench have been used to terminate the reaction. An example of a device to quench cracked effluent is U.S. Pat. No. 3,910,347 (Woebcke).
It is well known that in the process of cracking hydrocarbon the reaction temperature and reaction residence time are the two primary variables affecting severity, conversion and selectivity. Severity is related to the intensity of the cracking reactions, conversion is defined as the percent of feed transformed into a product, and selectivity is the degree to which the converted products constitute ethylene. Selectivity is generally measured as a ratio of ethylene to methane in the product gas effluent.
At low severity, selectivity is high, but because conversion is low, it is uneconomical to utilize low severity operation. Low severity operation is conducted generally at temperatures between 1200.degree. and 1400.degree. F. and residence times between 200 and 1000 milliseconds. High severity and, hence, high conversion may be achieved at temperatures between 1500.degree. and 2000.degree. F. However, selectivity is poor unless the high severity reaction can be performed at residence times below 200 milliseconds, usually between 20 and 100 milliseconds. At these very low residence times selectivities between 2.5 and 4.0 pounds of ethylene per pound of methane can be achieved, and conversion is generally over 95% by weight of feed. High severity operation, although preferred, has not been employed widely in the industry because of the physical limitations of conventional fired heater reactors. One of the limitations is the inability to remove heat from the product effluent within the allowable residence time parameter. For this reason most conventional systems operate at conditions of moderate severity, temperatures being between 1350.degree. and 1550.degree. F. and residence times being between 200 and 500 milliseconds. Although conversion is higher than at the low severity operation, selectivity is low, being about two pounds of ethylene per pound of methane. But because conversion is higher, the actual yield of ethylene is greater than obtained in low severity operation.
By using short residence time at high severity conditions it is possible to achieve selectivities of about 3:1 or greater. A number of processes have been developed which offer high severity thermal cracking. For example, furnaces have been developed which contain a large number of small tubes wherein the outlet of each tube is connected directly to an individual indirect quench boiler. This process has the disadvantage of being capital intensive in that the quench boiler is not common to a plurality of furnace tube outlets. Further, the high temperature waste heat must be used to generate low temperature, high pressure steam which is not desirable from a thermal efficiency viewpoint. Finally, high flue gas temperatures must be reduced by generation of steam in the convection section of the heater, again limiting the flexibility of the process.